The capacity of the immune system to respond to pathogen challenge or vaccination decreases dramatically with age. Since the initiation of T cell responses to newly encountered antigens during infection or vaccination is dependent on a diverse repertoire of T cells, we hypothesize that impaired immune function in the aged is due, in part, to declining T cell diversity. To address this hypothesis, we are exploiting a well- developed mouse model of influenza virus to assess the impact of declining immune function on the effective response to pathogen challenge. Our preliminary data confirm that there is an age-associated reduction in repertoire diversity among naove CD8 T cells. In addition, we have made the seminal observation that there is a preferential loss of the ability of aged mice to respond to specific influenza virus epitopes and that this correlates with a low precursor frequency of T cells for these epitopes. This results in the development of "holes in the repertoire" following de novo influenza virus infection, which correlated with impaired protective immunity assessed by heterosubtypic challenge. Clearly, these findings have profound implications for the development of rational vaccination strategies for the elderly. However, the extent to which reduced repertoire diversity directly impacts viral clearance is not known. We also don't know whether there are similar age-associated perturbations of the CD4 repertoire, and to what extent this impacts CD4 responses, as well as the generation and maintenance of functional CD8 memory. The Specific Aims are directed toward answering these questions. The proposed studies will advance the field by enhancing our understanding of the impact of aging on the repertoire and reactivity of CD4 and CD8 T cells. The data generated will have important implications for the development of effective vaccines for the elderly. PUBLIC HEALTH RELEVANCE: The capacity of the immune system to respond to pathogen challenge or vaccination decreases dramatically as we age. Elderly individuals are significantly more susceptible to infections than the young and notoriously difficult to successfully vaccinate. Consequently, respiratory virus infections, such as those mediated by influenza virus, are a major cause of death and hospitalization in the elderly. The studies proposed in the current application will determine the mechanisms underlying decreased immune responsiveness in a mouse model of influenza virus infection. The data generated will provide essential information for the development of new vaccination strategies suitable for aged individuals.